powerpc/64s/exception: Merge EXCEPTION_PROLOG_COMMON_2/3

Merge EXCEPTION_PROLOG_COMMON_3 into EXCEPTION_PROLOG_COMMON_2.

No generated code change except BUG line number constants.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190802105709.27696-29-npiggin@gmail.com